REPAIR AND MAINTENANCE OF BALCONIES

Transcription

1 SERVICES Property Condition Assessments Mech. & Elect. Engineering Structural Engineering Environmental Engineering Material Testing Construction Quality Assurance Reserve Fund Studies Capital Expenditure Plans Condominium Assessments Design & Project Management Rehabilitation Administration Contract Administration Temporary Structures Sustainability Hazardous Materials Building Science Geotechnical Engineering Forensic Services Litigation Support Repair and Maintenance of Balconies All balconies require regular maintenance which, properly executed should prolong it s economic service life and makes less frequent the expensive, often noisy and intrusive repairs that become necessary from time to time. Balcony slabs must be maintained in a state of good repair for safety reasons. Another consideration is the effect of deteriorated balcony slab edges on the appearance of a building. Design Balcony floor slab may consist of: Reinforced cast in place concrete Post tensioned concrete Precast, pre-stressed (hollow core) concrete panels Open web steel joists with metal pans and a concrete topping Timber joists and planks Concrete The concrete utilized in the construction of exterior balconies should be air entrained and free of salt. It is important that balcony slabs are well drained to ensure that the top surfaces are not saturated with water during periods of freezing weather. If the concrete is not air entrained then the top surface of the slab will gradually crumble and disintegrate due to freezing and thawing of the saturated concrete in the areas where ponding of water occurs. This type of deterioration also occurs under balcony carpets due to concrete surface being saturated by water trapped by the carpet during periods of cold weather. Salt in the concrete in more than very small amounts will cause the reinforcing steel to rust which leads to spalling and delamination of the concrete. Reinforced Reinforced concrete balcony slabs consist of reinforcing steel embedded in poured in place concrete. They are normally an extension of the floor slab inside the building and may be cantilevered out from the face of the building or supported at each end by concrete shearwalls. Sometimes the balcony slabs are supported by load bearing masonry walls. Much of the poured-in-place concrete forming the balcony slabs in our older buildings does not contain the entrained air necessary for freeze thaw durability and many of the balcony slabs constructed in winter time had more than enough salt added to the concrete at the time of mixing to cause rusting (galvanic corrosion) of the reinforcing steel when the concrete gets wet. For these reasons, areas of ponding on this type of slab should be eliminated and carpeting discouraged. Post-Tensioned Many apartment buildings constructed since the 1970 s have post tensioned concrete floor and balcony slabs. The difference between this type of slab and a reinforced concrete slab lies in the type of reinforcement. Instead of steel bars embedded in the concrete, the main reinforcement in a post tensioned slab are steel tendons (wire rope) with a diameter of approximately 15mm which pass through the slab inside plastic sleeves. The tendons are anchored at their ends. The construction procedure followed is that after the concrete has hardened the tendons are stretched by as much as 200mm to 400mm and

2 the ends then secured at the slab edges by steel wedges. This squeezes the concrete together and makes the slabs capable of supporting both themselves and the loads for which they are designed. In addition to the steel tendons, there is also a small amount of reinforcing steel included in the slab mainly to control drying shrinkage of the concrete. The advantage of this type of slab is that is it thinner than a conventionally reinforced slab and is also generally crack free due to the steel tendons squeezing the concrete slab and putting it in compression. The concrete forming these slabs should also be air entrained and free of salt with the top surface well drained. A special maintenance requirement with this type of slab is at the anchors holdings the ends of the tendons at the balcony slab edges. It is essential that these anchors be kept dry and it is of absolute importance that no salt (or salty water) be allowed to penetrate through the concrete to the anchor assembly since this could cause corrosion of the tendon end and wedges. Sometimes salty water manages to flow into the plastic sheath surrounding a tendon where it is then trapped causing gradual corrosion of the tendon until it fails. Failure is similar to a stretched elastic band snapping but with a much greater force. It is good maintenance practice to regularly monitor the condition of the tendons and anchors and to carry out repair work when required before tendons begin to fail. Precast Prestressed Hollow Core Concrete Panels Pre-tensioned concrete panels are manufactured in fabrication plants and incorporated into the building during construction. They are normally 200mm to 250mm thick contain from two to six voids, depending on the width of the panel, which extend through the entire length of each panel. The reinforcing consists of strands of high tensile steel about 8mm in diameter which is stretched between the two ends of a form and concrete poured around them. As the concrete sets, it bonds to the stands. Once the concrete has hardened, the ends of the stands are released and this puts the concrete in compression and gives the panels the ability to support both their own weight and the loads they are designed to carry. There are normally strands in both the top the top and the bottom of a panel with the strands being located between the voids and at the edges. The difference between post tensioned concrete and precast, prestressed concrete are: The tendons in post-tensioned concrete are separated from the concrete by a plastic sheath and are only held at the ends while the strands in a precast prestressed concrete panel are fully bonded to the concrete along their length. Precast pre-stressed concrete panels are relatively narrow such that there are joints between them which are visible in the balcony slab. The precast pre-stressed concrete panels either from the balcony slab cantilevering out from the face of the building in which case the joints between adjacent panels are at right angles to the building face or the ends are supported on shearwalls which project from the face of the building in which case the joints are parallel to the building face.

3 The special concerns that must be addressed with this type of balcony slab are: Where the slabs are cantilevered out from the face of the building continuous voids facilitate the movement of cold air in wintertime into those parts of the voids that are inside the building creating cold areas of floor and ceiling. This can be prevented by injecting foamed insulation into the voids to block them along the line of the wall. The concrete forming the top and bottom sections of panel is relatively then, about 25mm thick over the voids. Unsealed joints between panels extending out from a building are potential sources of water penetration into the building besides letting water drip onto the balcony below. Care should be taken to ensure that the top surface of the slab is sealed at these locations. The presence in the panels of both the prestressed strands and the voids makes repair work more difficult. Precast concrete panels containing conventional reinforcing steel have also been used to form the floor slabs and balconies of apartment buildings. They are normally 200mm to 250mm in thickness and contain two or three voids, which extend the entire length of the panel. The presence of the voids in the panel reduces the concrete protection provided to the reinforcing steel therefore consideration should be given to protecting the exposed surfaces of the precast concrete panels to prevent water penetration. Open Web Steel Joists (Hambro Joists) Open Web Steel Joists, often referred to as the Hambro system, also has its own unique forms of deterioration and related concerns. Lattice type (open web) steel joints supported at each end by load bearing masonry or concrete walls supported plywood or steel pans onto which concrete is placed normally to a depth of approximately 100mm to form the floor slab. The tops of the joists are embedded in the concrete which is lightly reinforced with welded wire mesh. Balcony slabs are an extension of the floor slabs with the top chords of open web joists being cantilevered out from the face of the building. Deterioration normally takes the form of corrosion of the reinforced top chord of the joists causing delamination of the surrounding concrete. This type of floor slab construction was popular in the greater Toronto area prior to the 1970 s and is still popular in many smaller communities. Repair cost associated with this type of balcony slab is often unexpectly high due to the extent of the work involved in strengthening the joists where they have rusted. Timber Timber balconies normally consists of timber planks supported by timber joists, one end of which are supported by the timber framing of the building and the other end by a timber or steel beam which in turn is supported by masonry columns or steel or timber posts. The enemy of timber balconies is wood rot which commonly develops at the sections of joist embedded in the brickwork and at joints between adjacent pieces of timber which remain damp for extended periods of time after rain. Wood rot also develops at other locations where the wood remains damp. To avoid wood rot, the joints between the joists and the brickwork should be sealed with caulking material. All other joint locations should be sealed as required to prevent water penetration. The exposed wood surfaces should be regularly coated with a preservative and in new construction pressure treated wood should be considered.

4 Size of Balcony Enclosures The safety of balconies is paramount importance and this goes beyond the slabs to the size of the balcony enclosures. Ontario Building Code (OBC) requirements regarding both the minimum height of balcony enclosure and the maximum size of openings in the enclosures have changed such that many of the older buildings no longer comply. In most jurisdictions it is not mandatory to modify balcony enclosures to bring them into compliance with the current requirements of the Building Code. Some local authorities have enacted By-laws requiring compliance. Normally extensive balcony repair work may trigger off a requirement for the enclosure s to be modified. This requirement is stated in section of the Building Code. Carpets and Coatings Many unit owners improve the appearance of their balcony slabs by installing decorative finishes ranging from carpeting to a coat of paint to ceramic tiles. Any finish that is impermeable to the passage of water will enhance the durability of the slab and improve its performance. Carpeting however, not only lets rainwater through but also holds it on the surface of the concrete. This creates problems for the slab surface and possibly for the reinforcing steel buried in the slab. Balcony concrete must be both air entrained and generally free of slat to be durable both is it has water ponding on the surface during warm weather or if it is subject to cycles of freezing and thawing while saturated. Both of these conditions are encouraged by the presence of carpeting. Much of the concrete forming the balconies in our older buildings does not contain the entrained air necessary for freeze/thaw durability and many of the older balcony slabs constructed in wintertime contain enough salt added to the concrete as an accelerator to cause rusting of the steel reinforcement if the concrete gets wet. Also the concrete slab of balconies facing busy streets often becomes contaminated with salt in spray blown from the roads in wintertime. If the concrete is not air entrained the surface of the slab immediately underneath the carpet may gradually spall and disintegrate due to freeze/ thaw cycling combined with water trapped by the carpet. Special attention must be given to balcony slabs covered with tiles to ensure that they are kept free of cracking. Otherwise water penetrating through cracked tiles and mortar joints will saturate the slab below, causing damage when it freezes. Durability To achieve durability and to minimize the amount of balcony slab maintenance, the following conditions are desirable. Good top surface drainage with no ponding. The top surface and outside faces of the slab sealed with a watertight coating or membrane ( a coat of breathing type of paint or a clear sealer is better than nothing). The soffit of the slab sealed with a breathing type of paint. Concrete air entrained No salt in the concrete Good concrete cover over reinforcing steel

5 Ice Accumulation on Sloped Roofs A common winter problem with sloped roofs is accumulation of ice at eaves. When this ice causes water to back up onto the roof it is commonly referred to as Ice Damming and in some instances can result in significant water damage as this water penetrates the roof. These ice accumulations are generally the result of snow melting on higher portions of the roof slope over the attic and the melt water freezing once it reaches colder portions of the roof slope, such as over the vented soffit. With the increasingly complex roof configuration incorporating multiple slope and dormers on many new buildings the configuration of roofs is becoming a greater factor in the ice damming equation. On a complex roof, portions of the roof will receive sun while other segments are shaded and colder. Snow melting on the sunny portion of the slope melts and refreezes on the shaded lower portion. Correction of ice damming involves two aspects of work. The first is to reduce the conditions under which snow on the roof melts. This can be accomplished by reducing the attic temperature by increasing ventilation of the attic or increasing insulation levels in the ceiling. The second aspect of work is to ensure melt water can drain and does not re-freeze on colder portions of the roof. On a complex roof this may require the installation of heating cables to ensure paths of drainage are open.

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